Fig. 8. Neural lineage formation in the larval central brain of wild-type, brat and pros mutant clones. (A) In wild-type Drosophila, neural progenitor cells are required for lineage formation: neuroblasts (NB) divide asymmetrically to self-renew and to produce a series of ganglion mother cells (GMC); GMCs instead differentiate by undergoing a single terminal division that produces two postmitotic ganglion cells (GCs). GCs send out axons contributing to fibre tracts. (B) Somatic mutation of the tumour suppressor brat, and loss of function of the cell fate determinant pros impede differentiation of progenitor cells into GCs. Instead, these mutant cells retain progenitor cell-like characteristics and indefinitely self-renew, thereby generating clonally derived brain tumours. Targeted misexpression of either brat or pros in brat mutant cells restores differentiation which abrogates brain tumour formation.